seismic capacity evaluation of unreinforced masonry residential

12
Nat. Hazards Earth Syst. Sci., 12, 3753–3764, 2012 www.nat-hazards-earth-syst-sci.net/12/3753/2012/ doi:10.5194/nhess-12-3753-2012 © Author(s) 2012. CC Attribution 3.0 License. Natural Hazards and Earth System Sciences Seismic capacity evaluation of unreinforced masonry residential buildings in Albania H. Bilgin and O. Korini Department of Civil Engineering, Epoka University, Tirana, Albania Correspondence to: H. Bilgin ([email protected]) Received: 4 September 2012 – Revised: 18 November 2012 – Accepted: 20 November 2012 – Published: 19 December 2012 Abstract. This study evaluates seismic capacity of the un- reinforced masonry buildings with the selected template de- signs constructed per pre-modern code in Albania consider- ing nonlinear behaviour of masonry. Three residential build- ings with template designs were selected to represent an im- portant percentage of residential buildings in medium-size cities located in seismic regions of Albania. Selection of tem- plate designed buildings and material properties were based on archive and site survey in several cities of Albania. Ca- pacity curves of investigated buildings were determined by pushover analyses conducted in two principal directions. The seismic performances of these buildings have been deter- mined for various earthquake levels. Seismic capacity eval- uation was carried out in accordance with FEMA (Federal Emergency Management Agency) 440 guidelines. Reasons for building damages in past earthquakes are examined using the results of capacity assessment of investigated buildings. It is concluded that of the residential buildings with the tem- plate design, with the exception of one, are far from satisfy- ing required performance criteria. Furthermore, deficiencies and possible solutions to improve the capacity of investigated buildings are discussed. 1 Introduction Masonry is the most important construction material in the history of humankind. This term is used in a variety of forms such as stone, clay brick, cellular concrete block and adobe for the construction of building structures. The combination of heavy weight and high stiffness along with the lower ten- sile strength makes masonry structures prone to earthquakes. Since many urban settlements are in located in moderate to severe seismic zones of the world, seismic vulnerability assessment of masonry buildings requires special consider- ation. Even though a large percentage of loss of life during the past earthquakes have occurred due to the poor perfor- mance of masonry buildings, the efforts to measure and to in- crease their seismic performance are not adequate when com- pared with current advances in the area of reinforced concrete structures (Tomazevic, 1999; Erberik and Yakut, 2008) Recent devastating earthquakes have emphasised the inad- equate seismic performance of unreinforced masonry (URM) buildings to the worldwide community. In literature, sev- eral studies related to performance of URM buildings in past earthquakes are available (Calvi, 1999; Decanni et al., 2004; Jagadish et al., 2003; Kaplan et al., 2010; Klingner, 2006; Pasticier et al., 2008; Yilmaz et al., 2012; Yoshimura and Kuroki 2001). Many of URM buildings were affected by se- vere earthquakes due to poor quality of construction, poor workmanship, aging and the lack of maintenance. Following observed damages in past earthquakes (i.e. 1999 Kocaeli and Duzce in Turkey, 2001 Gujarat, India; 2002 Molise and 2009 L’Aquila, Italy; 2010 Haiti and 2010 Chile), there have been significant efforts to mitigate the earthquake hazards on URM buildings in many coun- tries (Decanni et al., 2004; Jagadish et al., 2003; Kaplan et al., 2010; Klingner, 2006; Lagomarsina and Penna 2003; Yoshimura and Kuroki 2001). Seismic safety of URM build- ings has been questioned in the wake of L’Aquila, Italy (6 April 2009), Haiti (12 January 2010) and Chile (27 Febru- ary 2010) earthquakes because there was a widespread con- viction that these buildings experienced considerable damage compared to reinforced concrete buildings. In Albania, template designs developed by the govern- mental authorities are used for many of the buildings in- tended for residential purposes as common practice to save on architectural fees and ensure quality control during the Published by Copernicus Publications on behalf of the European Geosciences Union.

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Page 1: Seismic capacity evaluation of unreinforced masonry residential

Nat. Hazards Earth Syst. Sci., 12, 3753–3764, 2012www.nat-hazards-earth-syst-sci.net/12/3753/2012/doi:10.5194/nhess-12-3753-2012© Author(s) 2012. CC Attribution 3.0 License.

Natural Hazardsand Earth

System Sciences

Seismic capacity evaluation of unreinforced masonry residentialbuildings in Albania

H. Bilgin and O. Korini

Department of Civil Engineering, Epoka University, Tirana, Albania

Correspondence to:H. Bilgin ([email protected])

Received: 4 September 2012 – Revised: 18 November 2012 – Accepted: 20 November 2012 – Published: 19 December 2012

Abstract. This study evaluates seismic capacity of the un-reinforced masonry buildings with the selected template de-signs constructed per pre-modern code in Albania consider-ing nonlinear behaviour of masonry. Three residential build-ings with template designs were selected to represent an im-portant percentage of residential buildings in medium-sizecities located in seismic regions of Albania. Selection of tem-plate designed buildings and material properties were basedon archive and site survey in several cities of Albania. Ca-pacity curves of investigated buildings were determined bypushover analyses conducted in two principal directions. Theseismic performances of these buildings have been deter-mined for various earthquake levels. Seismic capacity eval-uation was carried out in accordance with FEMA (FederalEmergency Management Agency) 440 guidelines. Reasonsfor building damages in past earthquakes are examined usingthe results of capacity assessment of investigated buildings.It is concluded that of the residential buildings with the tem-plate design, with the exception of one, are far from satisfy-ing required performance criteria. Furthermore, deficienciesand possible solutions to improve the capacity of investigatedbuildings are discussed.

1 Introduction

Masonry is the most important construction material in thehistory of humankind. This term is used in a variety of formssuch as stone, clay brick, cellular concrete block and adobefor the construction of building structures. The combinationof heavy weight and high stiffness along with the lower ten-sile strength makes masonry structures prone to earthquakes.Since many urban settlements are in located in moderateto severe seismic zones of the world, seismic vulnerability

assessment of masonry buildings requires special consider-ation. Even though a large percentage of loss of life duringthe past earthquakes have occurred due to the poor perfor-mance of masonry buildings, the efforts to measure and to in-crease their seismic performance are not adequate when com-pared with current advances in the area of reinforced concretestructures (Tomazevic, 1999; Erberik and Yakut, 2008)

Recent devastating earthquakes have emphasised the inad-equate seismic performance of unreinforced masonry (URM)buildings to the worldwide community. In literature, sev-eral studies related to performance of URM buildings in pastearthquakes are available (Calvi, 1999; Decanni et al., 2004;Jagadish et al., 2003; Kaplan et al., 2010; Klingner, 2006;Pasticier et al., 2008; Yilmaz et al., 2012; Yoshimura andKuroki 2001). Many of URM buildings were affected by se-vere earthquakes due to poor quality of construction, poorworkmanship, aging and the lack of maintenance.

Following observed damages in past earthquakes (i.e.1999 Kocaeli and Duzce in Turkey, 2001 Gujarat, India;2002 Molise and 2009 L’Aquila, Italy; 2010 Haiti and2010 Chile), there have been significant efforts to mitigatethe earthquake hazards on URM buildings in many coun-tries (Decanni et al., 2004; Jagadish et al., 2003; Kaplanet al., 2010; Klingner, 2006; Lagomarsina and Penna 2003;Yoshimura and Kuroki 2001). Seismic safety of URM build-ings has been questioned in the wake of L’Aquila, Italy(6 April 2009), Haiti (12 January 2010) and Chile (27 Febru-ary 2010) earthquakes because there was a widespread con-viction that these buildings experienced considerable damagecompared to reinforced concrete buildings.

In Albania, template designs developed by the govern-mental authorities are used for many of the buildings in-tended for residential purposes as common practice to saveon architectural fees and ensure quality control during the

Published by Copernicus Publications on behalf of the European Geosciences Union.

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3754 H. Bilgin and O. Korini: Seismic capacity evaluation of unreinforced masonry residential buildings

communist period (Korini, 2012). The representative typol-ogy of the country corresponds to URM buildings. There arestandard URM buildings all over the country for residentialpurposes, 5 stories with different plans. The majority of ex-isting residential masonry buildings in Albania, like in manyother European countries, has been designed considering ear-lier codes (KTP-9, 1978; KTP-N2, 1989) when seismic loadswere not required or the design was to lower level of seismicloads of what is currently specified.

Masonry systems can be either engineered or non-engineered and be classified as unreinforced, confined andreinforced masonry. Each system has been built through dif-ferent construction technologies and exhibit different earth-quake responses; URM exhibits brittle failure whereas theother two have enhanced strength and ductility. This studyaims to evaluate seismic capacity of the existing masonry res-idential buildings constructed per pre-modern seismic code(KTP-9, 1978) in Albania considering nonlinear behaviourof URM components. Three residential buildings with tem-plate designs constructed in accordance with Albanian Code(KTP-9, 1978) were selected to represent an important per-centage of existing residential buildings in Albania of mod-erate seismicity. Selection of template designed buildingsand material properties were based on field investigation,archive study and the blueprints of these structures. Mechan-ical characteristics for the case buildings were taken fromtheir blueprints and adopted for the analysis. Capacity curvesof investigated buildings were determined by pushover anal-yses conducted in two principal directions. Seismic capacityevaluation is carried out in accordance with FEMA (FederalEmergency Management Agency) 440 (FEMA 440, 2004).Reasons for building damages related to URM in past earth-quakes are examined using the results of capacity assessmentof investigated buildings. Deficiencies and possible solutionsto improve the capacity of URM residential buildings are dis-cussed.

2 Description of structures

Until the end of communist period in 1990s, masonry build-ings continued to be built using template designs. Masonrywas used for public and governmental buildings as a lowcost construction method for that time. Today these build-ings are still in use and the main functions are mostly for res-idential purposes. Hence, a considerable number of buildingshave the same template designs in different parts of Albania(Korini, 2012).

A field and archive survey were carried out in Tirana cityto select the most common template designs among residen-tial buildings. Being the capital city of Albania, Tirana repre-sents a medium-size city in a seismic part of Albania (Aliajet al., 2010). According to survey results, there were about30 types of residential buildings with template designs. Itis observed that the most common templates are TD-83/3,

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0.190.19

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0.91

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.9x2.1

D-0

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D-0

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D-0

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D-0

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1.2

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ney

blo

ck

Chim

ney

blo

ck

Chim

ney

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ck

C1

C1

C1

C1

C1

C1

C1

C1

ABC

4.44.40.38

3.950.383.950.38

9.04

D-0.9x2.1

D-0.9x2.1

Figure 1. Template design 83/3, (National Archive of Albania, 1983)

Fig. 1.Template design 83/3, (National Archive of Albania, 1983).

TD-72/3, and TD-72/1 which covers nearly 15 % of the totalbuilding stock (Korini, 2012). According to the blueprints ofeach template design, selected buildings are built with claybricks of M75 with a resistance of 7.5 MPa and mortar ofM25 with a resistance of 2.5 MPa. These mechanical proper-ties taken from the blueprints of respective template designsare used and adopted for the analysis. Unlike many residen-tial reinforced concrete buildings, URM buildings generallyhave a uniform distribution of mass and stiffness in horizon-tal and vertical plane because of similar architectural featuresdue to similar purpose of use in all storeys. Therefore, theyare less prone to structural irregularity effects such as, heavyoverhangs, great eccentricities between mass and stiffnesscentres, etc. All of them have five floors. The load bearingwall thickness is 380 mm on first two storeys and 250 mm onthe remaining three storeys. Representative plan views of thethree buildings for the ground story are shown in Figs. 1–3.All dimensions are in m.

Nat. Hazards Earth Syst. Sci., 12, 3753–3764, 2012 www.nat-hazards-earth-syst-sci.net/12/3753/2012/

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H. Bilgin and O. Korini: Seismic capacity evaluation of unreinforced masonry residential buildings 3755

18

1 2 4 5 6

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E

D

B

A

87

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W-1.5x1.2

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D-0.9x2.1

D-0

.9x2.1

D-0

.9x2.1

Figure 2. Template design 72/3, (National Archive of Albania, 1983)

Fig. 2.Template design 72/3, (National Archive of Albania, 1983).

3 Mathematical modelling of representative buildings

SAP2000 (CSI, 2011) program has been used for modellingthe considered building typologies. The 3-D modelling ofURM buildings starts from the hypotheses on their struc-tural and earthquake behaviour; the load bearing structureunder horizontal and vertical loadings is defined, with wallsand floors. The walls are the load bearing elements, whilethe floors are considered as planar stiffening elements (rigiddiagrapham), on which the horizontal effects are distributedbetween the walls connected. Presence of ring beams in ma-sonry prevents out-of-plane failure (Magenes, 2010) and pro-vides the development of global structural behaviour gov-erned by in-plane response of walls. This fact was also ob-served in previous shaking table tests (Benedetti et al., 1998;Mazzon et al., 2009). Experimental tests on masonry infilledconcrete frames (Fardis, 1997) have revealed that severe ac-celeration levels are required to trigger an out-of-plane col-lapse due to increased natural period of vibration of thepanel. In this study, the local flexural response of floors andout-of-plane behaviour of walls are not computed since theyare considered as negligible with respect to the global build-ing response dominated by their in-plane response.

For the modelling of the selected buildings two typesof issues should be considered: correct representation ofthe mathematical model and inelastic characteristics of

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Figure 3. Template design 72/1, (National Archive of Albania, 1983) Fig. 3.Template design 72/1, (National Archive of Albania, 1983).

materials. URM is a composite construction material whichconsists of masonry units and mortar. Brick and stone are theusual elements of masonry units. Mortar is used to make theconnection between these units. Under vertical and horizon-tal loads, load-bearing of masonry considered as the assem-blage of the masonry units and mortar is influenced by thecompressive, shear and flexural strengths, durability, waterabsorption and thermal expansion.

To model this anisotropy, two different approaches havebeen offered in literature: “micro modelling” and “macromodelling”. Each modelling technique requires the adoptionof different constitutive models. Modelling of masonry dueto its anisotropic behaviour has been a very difficult task forseveral years. As a first approach, the finite elements meth-ods can be used to model the masonry constitutive elements(mortar and unit elements). They are discretized into a certainnumber of finite elements then suitable constitutive nonlinearlaws are adopted. A second approach is based upon the jus-tification of “equivalent frames”. The structure is describedby an assemblage of piers and spandrel elements. These ele-ments are connected by rigid offsets and modelled by properconstitutive laws in order to take into account the mechanicalnonlinearity (Dolce, 1989). Several studies have been doneby different researchers; (Lagomarsino et al., 2006; Gam-barotta and Lagomarsino, 1997; Penelis, 2006; Calderini andLagomarsino, 2008; Belmouden, 2009). In these approaches,

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3756 H. Bilgin and O. Korini: Seismic capacity evaluation of unreinforced masonry residential buildings

20

a) Four nodded shell element b) In-plane stresses

Figure 4. A four nodded shell element and in plane stresses. (SAP2000 reference manual)

s 11

s 11

s 22 s 22

s 12

s 12

Fig. 4. A four nodded shell element and in plane stresses.(SAP2000 reference manual).

a nonlinear macro-element model, able to reproduce earth-quake damage to masonry structures and failure modes ob-served during experimental testing, has been implementedwith similar approximations.

For nonlinear analysis of the selected URM residentialbuildings, material properties determined from the blueprintsof the designs were taken into account. As aforementioned,many of the buildings intended for residential services havesimilar construction procedure supervised by governmentalauthorities. Material properties considered in this study weredetermined based on an archive study of 30 buildings.

4 Nonlinear material properties

Pushover analyses have been performed using SAP2000Nonlinear Version 15 (CSI, 2011) that is a general-purposestructural analysis program. Member sizes in the templatedesigns were used to model the selected buildings for nonlin-ear analysis. No simplifications are made for members; likerounding-off or grouping members with close properties. Allstructural elements are modelled as given in the template de-sign. Three-dimensional model of each building is created inSAP2000 to perform pushover analysis. Walls are modelledas nonlinear layered shell elements. The anisotropy of ma-sonry is modelled by two different stress–strain curves. Eachof them represents respectively vertical stresses S11, hori-zontal stress S22 and shear stress S12 (Fig. 4). S11 and S22stress–strain curves are determined using the relation pro-vided by Kaushik et al. (2007) (Fig. 5). Parabolic part of thecurve is predicted by Eq. (1) which is valid until stress dropsto 0.9fm in the descending part.

fm

f′

m= 2

εm

ε′

m−

(εm

ε′

m

)2

(1)

f′

m = 0.63.f 0.49b f 0.32

j (2)

wherefm is the mortar compressive strength,f′

m is the ma-sonry compressive strength,εm is strain in masonry andε

mis the peak strain corresponding tof

m. In Eq. (2),fb andfj ; brick compressive strength according to Eurocode 6 (EN

21

Figure 5. Masonry idealized stress-strain curve for compression (Kausnik et al., 2007)

Fig. 5. Masonry idealised stress–strain curve for compression(Kaushik et al., 2007).

1996-1, 2005) and mortar compressive strength, respectively.Then a linear part is proposed for the curve. It is assumed thatmasonry may have a residual strength at 20 %, due to frictionbetween detached parts. In this study, the Eq. (3) is used toestimateε

m.

ε′

m = C′

j ×f

m

E0.7m

(3)

C′

j =0.27

f 0.25j

(4)

On the other hand, to take into account the shear resis-tance, shear stress–strain curve should be defined. This curveneeds to represent the horizontal failure. In reality, when amasonry member is subjected to lateral ground motion thehorizontal resisting strength is represented by the cohesionand friction between brick and mortar which can be ex-pressed with Coloumb friction (Eq. 5):

τ = c + σ × tgϕ (5)

whereσ is the vertical stress andtgϕ stands for friction be-tween elements. In this study, shear resistance is representedby a material nonlinear curve (cohesion) and the friction isneglected. Annex C of EN 1998-3 (EC-8) provides drift lim-its for in-plane response of existing URM buildings. For theS12 stress–strain material curve, an approach which was usedby researchers (Lagomarsino et al., 2006, 2007; Korini andBilgin, 2012) have been taken into consideration and adoptedfor the analysis.

5 Seismic demand

Seismic loads are commonly represented by response spec-trum functions which are derived from time history recordsof earthquakes in specific areas. Albania, located in theBalkan Peninsula, has a moderate seismic hazard and tec-tonic activity (Aliaj et al., 2010). Microzonation of the coun-try allowed classifying the soil of the country in three types.

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H. Bilgin and O. Korini: Seismic capacity evaluation of unreinforced masonry residential buildings 3757

22

0.0

0.2

0.4

0.6

0.8

1.0

0 0.4 0.8 1.2 1.6 2

Accele

rati

on

(g

)

Period (sec)

EC-8; Soil category C; PGA= 0.20 g

KTP-89; Soil category II; PGA=0.22 g

Figure 6.Comparison of elastic response spectrums

Fig. 6.Comparison of elastic response spectrums.

KTP-N2-89 normative design response spectrums are stillused, since Eurocode 8 is not yet legally approved. In thisstudy both Eurocode 8 and KTP-N2-89 spectrums are usedin order to make a comparison and question the adequacy ofcurrent design spectrum. It is obvious that the Eurocode 8spectrum has higher demand than the other (Fig. 6). Sincethe following existing structures have been constructed indifferent parts of the country, both ground conditions andseismicity is variable. In this study, the demand calculationsfor the seismic assessment of the considered buildings areperformed considering the soil Type C with a moderate seis-micity (0.2 g) according to Eurocode 8 (2004) and its cor-responding spectra considering soil category II and mediumseismicity (0.22 g) in KTP-N2-89 (1989).

6 Identification of damage limit states

A performance level is a limit state on the pushover curvethat is used to classify the damage. As recommended by sev-eral researchers (e.g. Priestley, 1997), deformation thresh-olds may be the best indicators of identifying the limit statescorresponding to structural and non-structural performancedamage levels. In order to define these damage limits orperformance levels of the URM template designs, there areneither experimental results based on laboratory tests noravailable values calibrated from observed damages duringthe earthquakes. On the other hand, values of the mechan-ical properties of the materials used in these template de-signs have been taken from the blueprints of these projectsand the actual values are not completely known. Consideringall these aspects, there are different approaches to damagelimit states classification for URM. Calvi (1999) and Lago-marsino and Penna (2003) have introduced different thresh-olds of the spectral displacement for discrete damage statesbased on the bilinear representation of the capacity spectrum.In this study for the performance assessment of the consid-ered template designs, both thresholds have been employed.

23

Sa

(g)

Sp

ectr

al a

cce

lera

tion

Sd (cm)Spectral displacement

No d

am

age

Slight

Moderate ExtensiveComplete

LS2

0.1% drift

DuLS3

0.3% drift LS4

0.5% drift

Figure 7.Performance levels on pushover curve

Fig. 7.Performance levels on pushover curve.

Calvi (1999) proposed four damage limit states for masonrystructures (Fig. 7). Lagomarsino and Penna (2003) identifiedyield point and ultimate displacement for a structure and thensplit the capacity curve into 5 parts. Following the outlinedcriteria (see Tables 1 and 2); the thresholds of the spectraldisplacements are obtained for the damage limit states.

7 Pushover analysis and capacity evaluation

The pushover analysis consists of the application of grav-ity loads and a representative lateral load pattern. Gravityloads were in place during lateral loading. In all cases, lateralforces were applied monotonically in a step-by-step nonlin-ear static analysis. The applied lateral forces were propor-tional to the product of mass and the first mode shape ampli-tude at each story level under consideration.

In pushover analysis, the response of structure is charac-terised by a capacity curve that represents the relationshipbetween the base shear force and the displacement of theroof. This useful demonstration is very practical and can eas-ily be visualised by practising engineers. Roof displacementis commonly used for capacity curve.

Capacity evaluation of the investigated residential build-ings is performed using damage limit states suggested byCalvi (1999). Pushover analysis data and criteria of Table 1were used to determine inter-storey drift ratios of each build-ing in both directions. Identification of damage limit statesand its representations on capacity curves for each buildingis given in Figs. 8–10b and d. Small displacement capacitiesat different performance levels are remarkable for the build-ings with greater openings in the respective directions dueto failure of masonry elements. Also, TD-83/3 x-directionand TD-72/3 in both directions do not have the expected dis-placement capacity due to lack of continuous walls (windowopenings) and irregularity in plans and elevations. The re-ductions in wall thickness cause a jump in inter-storey driftratios at the third floor as obviously seen below (Figs. 8–10).

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3758 H. Bilgin and O. Korini: Seismic capacity evaluation of unreinforced masonry residential buildings

Table 1.Performance levels and criteria provided by Calvi (1999).

Performance level Performance criteria

LS1 – No damage

LS2(Minor structural damage and/or moderatenon-structural damage)

– Structure can be utilised after the earthquake, without any need for significantstrengthening and repair to structural elements.– The suggested drift limit is 0.1 %.

LS3(Significant structural damage and extensivenon-structural damage)

– The building cannot be used after the earthquake without significant repair.Still, repair and strengthening is feasible.– The suggested drift limit is 0.3 %.

LS4(Collapse)

– Repairing the building is neither possible nor economically reasonable. Thestructure will have to be demolished after the earthquake. Beyond this LS globalcollapse with danger for human life has to be expected.– The suggested drift limit is 0.5 %.

24

(a) Inter-storey drifts for x- direction (b) Capacity curve in x- direction

(c) Inter-storey drifts for y- direction (d) Capacity curve in y- direction

Fig. 8. Inter-storey drift ratios and capacity curve representation of the TD-83/3 obtained by

pushover analysis

Fig. 8. Inter-storey drift ratios and capacity curve representation of the TD-83/3 obtained by pushover analysis.

The accurate prediction of inter-storey drift ratio and itsdistribution along the height of the structures is very criticalfor the seismic performance evaluation purposes since thestructural damage is directly related to this parameter. Theinter-storey drift ratios and their corresponding profiles alongthe height of the template designs are illustrated in Figs. 8–10a and c. As this is the case, the inter-storey drift ratio

over the height of the structures become non-uniform as wallthickness changes.

Pushover analysis data and criteria of Table 2 were usedto determine damage limit states according to Lagomarsinoand Penna (2003) in both directions. Identification of damagelimit states is given Table 3.

The displacement capacity values are solely not meaning-ful themselves. They need to be compared with displacement

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25

(a) Inter-storey drifts for x- direction (b) Capacity curve in x- direction

(c) Inter-storey drifts for y- direction (d) Capacity curve in y- direction

Fig. 9. Inter-storey drift ratios and capacity curve representation of the TD-72/3 obtained by

pushover analysis

Fig. 9. Inter-storey drift ratios and capacity curve representation of the TD-72/3 obtained by pushover analysis.

26

(a) Inter-storey drifts for x- direction (b) Capacity curve in x- direction

(c) Inter-storey drifts for y- direction (d) Capacity curve in y- direction

Fig. 10. Inter-storey drift ratios and capacity curve representation of the TD-72/1 obtained by

pushover analysis

Fig. 10.Inter-storey drift ratios and capacity curve representation of the TD-72/1 obtained by pushover analysis.

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3760 H. Bilgin and O. Korini: Seismic capacity evaluation of unreinforced masonry residential buildings

Table 2. Performance levels and criteria provided by Lagomarsinoand Penna (2003).

Damage state Spectral displacement,Sd

No damage Sd < 0.7DySlight 0.7Dy < Sd ≤ DyModerate Dy < Sd ≤ Dy + 0.25(Du − Dy)

Extensive Dy + 0.25(Du − Dy) < Sd ≤ DuComplete Sd > Du

27

Figure 11. Ultimate level of shear stress distribution for one wall element of TD-83/3 building obtained

from pushover analysis (x- direction - kPa)

Fig. 11. Ultimate level of shear stress distribution for one wall el-ement of TD-83/3 building obtained from pushover analysis (x-direction – kPa).

demand values. According to modern codes, residentialbuildings are expected to satisfy a life safety performancelevel which corresponds to LS3 according to Calvi (1999)under design earthquakes, corresponding to 10 % probabil-ity of exceedance in 50 yr (Aliaj et al., 2010). Performancepoint estimates and damage limit states of building capaci-ties corresponding to calculated performance levels are com-pared to see whether the residential buildings have adequatecapacity. Performance points of the structures were obtainedas described in FEMA 440. Table 4 lists performance levelsof each building under both KTP-N2-89 and EC8 spectrum.

The seismic performance assessment is made for the threetemplate design masonry buildings. For two of them the per-formance was insufficient and the risk of collapse was veryhigh. Significant damage (LS3) is caused in all the obtainedperformances. The Albanian and European seismic codesproduced different performance levels. European code wasmostly unfavorable as expected. Only the third structure maybe considered safe under the moderate seismic conditionstaken into account. Performance assessment results are sum-marised in Table 5.

8 Remarks on building responses

TD-83/3 shows a brittle behaviour in x-direction. This isprobably because of the greater area of openings in this direc-tion. Even though most of the masonry does not reach their

28

Figure 12. Ultimate level of shear stress distribution for one pier element of TD-83/3 building obtained

from pushover analysis (y- direction - kPa)

Fig. 12. Ultimate level of shear stress distribution for one pierelement of TD-83/3 building obtained from pushover analysis (y-direction – kPa).

ultimate shear capacity, some of the spandrel elements gofailure (Fig. 11). The performance point is obtained by usingFEMA 440 procedures only for KTP spectrum. RegardingEurocode 8 spectrum, performance is not reached becauseof insufficient capacity in this direction. The y-direction be-haviour is more ductile. Pier elements are more efficient(Fig. 12). The performance point is found under both spec-trums (Table 4). The performance is lower than LS3 damagelevel for this direction and the damage is moderate in case ofthis type of earthquake happens.

TD-72/3 has a very brittle behaviour in the x-direction.According to the pushover curve, it fails at the small range ofdisplacements. Due to greater openings and plan irregular-ity, the performance point is obtained only for KTP-N2-89spectrum with moderate damage (before LS3 level), but veryclose to collapse. The peripheral walls carry most of the loadand they fail while most of the masonry does not reach ulti-mate shear resistance. Figure 13 shows a peripheral wall andanother one with large openings at failure. The performancein y-direction is better than x- due to the efficiency of pier el-ements (Fig. 14). Performance point is found and LS3 levelis satisfied only for KTP spectrum (Table 4). Although theresponse in y-direction is more ductile, it is not adequate tosatisfy EC8 spectrum.

TD-72/1 shows a ductile behaviour in x-direction. The reg-ularity in plan and elevation makes a good distribution ofstresses and increases energy dissipation capacity (Fig. 15).LS3 damage limit state is assured. Due to the low lateral loadcapacity, extensive damage is expected under EC8 spectrum.Performance point is reached only for EC8 spectrum andit is close to LS3 (Table 4). This building has a higher ca-pacity and resistance in y-direction. Stresses are uniformly

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Table 3.Damage limit states according to Lagomarsino and Penna (2003).

Structure Type DirectionDamage Limit state thresholds (cm)

Slight Moderate Extensive Complete

TD-83/3X 0.53 0.75 1.13 2.20Y 0.70 1.00 3.20 9.80

TD-72/3X 0.63 0.90 1.28 2.40Y 0.56 0.80 1.40 3.20

TD-72/1X 0.42 0.60 1.58 4.50Y 0.56 0.80 2.13 6.90

Table 4.Performance points according to Fema 440 for the considered buildings under both spectrums.

Design spectrumPerformance points according to Fema 440

DirectionTD-83/3 TD-72/3 TD-72/1

Base shear Displacement Base shear Displacement Base shear Displacement(kN) (cm) (kN) (cm) (kN) (cm)

Eurocode 8x NA NA NA NA 1230 2.3y 1430 2.4 NA NA 1900 2.1

KTP-89x 1800 1.5 1630 1.8 NA NAy 1430 2.4 2410 1.7 1650 1.1

29

Figure 13. Ultimate level of shear stress distribution for a) a peripherial wall element; b) one wall with

opening of TD-72/3 building obtained from pushover analysis (x- direction - kPa)

Fig. 13. Ultimate level of shear stress distribution for(a) a periph-erial wall element;(b) one wall with opening of TD-72/3 buildingobtained from pushover analysis (x-direction – kPa)

distributed (Fig. 16) and global response is satisfactory. Per-formance is found for both spectrums with a medium tohigh ductility (Table 3). LS3 damage level is satisfied forboth spectrums. Moderate damage is caused by both of them,which is higher for EC8 seismic load. Also it is obvious thebig distance between the performance point and ultimate LS4damage level. This means that this structure is safe underboth spectrums in y-direction.

30

Figure 14. Ultimate level of shear stress distribution for TD-72/3 building obtained from pushover

analysis (y- direction - kPa)

Fig. 14. Ultimate level of shear stress distribution for TD-72/3building obtained from pushover analysis (y-direction – kPa).

9 Summary and conclusions

This study evaluates seismic capacity of residential build-ings with the selected template designs constructed per pre-modern code in Albania considering nonlinear behaviourof masonry. Three residential buildings with template de-signs were selected to represent an important percentageof residential building stock in mid-sized cities located inthe seismic region of Albania. Selection of template de-signed buildings and material properties were based on field

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3762 H. Bilgin and O. Korini: Seismic capacity evaluation of unreinforced masonry residential buildings

Table 5.Analysis results for all structure.

Building ID Direction KTP-N2-89 Albanian EC 8 Comment

TD-83/3x Safe LS3 Risky* Low stiffness according to Eurocodey Safe LS3 Safe LS3 Moderate damage under both spectrums

TD-72/3x Safe LS3 Risky* Low stiffness according to

Eurocode. Performance is close to collapse for KTPy Safe LS3 Risky* Low stiffness according to Eurocode but safe for KTP

TD-72/1x Safe LS3 Safe LS3 Low stiffness according to KTP but safe for ECy Safe LS3 Safe LS3 Moderate damage under both spectrums

* Risky means no performance is found due to low stiffness.

31

Figure 15. Ultimate level of shear stress distribution of TD-72/1 building obtained from pushover

analysis (x- direction - kPa)

Fig. 15.Ultimate level of shear stress distribution of TD-72/1 build-ing obtained from pushover analysis (x-direction – kPa).

investigation and survey in the governmental archives of Al-bania. Capacity curves of investigated buildings were deter-mined by pushover analyses conducted in two principal di-rections. Seismic performance evaluation was carried out inaccordance with Fema 440 provisions. Damage limit statesthresholds suggested by Calvi (1999) and Lagomarsino andPenna (2003) have been used. Reasons of building damagesin past earthquakes are examined using the results of capacityassessment of investigated buildings. Deficiencies and possi-ble solutions to improve the capacity of residential buildingsare discussed. The observations and findings of the currentstudy are briefly summarised in the following:

1. Based on archive investigations according to theblueprints of each template design, selected buildingsare built with clay bricks of M75 with a resistanceof 7.5 MPa and mortar of M25 with a resistance of2.5 MPa.

2. Evaluation of the capacity curves for the investigatedbuildings points out that those storeys where the thick-nesses of the walls are reduced may cause a deficiencyin the seismic performance. Deformation demands areconcentrated at the floor where the change occurs. Suchabrupt changes in stiffness and strength may lead to fail-ure at the level of change, since the load above and

32

Figure 16. Ultimate level of shear stress distribution of TD-72/1 building obtained from pushover

analysis (y- direction - kPa)

Fig. 16. Ultimate level of shear stress distribution of TD-72/1 build-ing obtained from pushover analysis (y-direction – kPa).

below the floor is similar. Observations on capacitycurves considering the damage limit states thresholdsgenerally being maximum in these stories (inter-storeydrift ratios) shows this fact.

3. Regarding the stress distribution and inter-storey driftratios, stress concentrations and inter-storey drifts arelumped at third story levels where a reduction from sec-ond storey to third storey was made. This type of suddenreduction in wall thickness cause deficiencies for the up-per part of the building as it is observed in this study. Ex-cessive inter-storey drift and inadequate shear strengthmay result in moderate to severe damage to these brit-tle structures. As a conclusion, wall thickness should bereduced in a gradual manner for new buildings.

4. Masonry shear walls are pierced by window and dooropenings. Above and below the opening, spandrels con-nect the walls. In direction where significant amount ofopenings, the capacity curves show the effects of dis-continuity in masonry. The observations on the templatedesigns indicated that although windows are located in

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both directions, openings are intensified in one orthog-onal direction (longitudinal). Considering the fact thatamount of openings is a significant parameter in theseismic performance of URM buildings, this situationcan cause notable differences in lateral strength and dis-placement capacities in two orthogonal directions andclearly be observed from the capacity curves. Althoughthe difference on lateral strength capacity is somewhatlimited, the difference in displacement capacity is note-worthy. The displacement capacity in direction wherethe number of openings is intensified is less than half ofthat other. This study shows that the considered amountof openings decreases energy dissipation capacity byaround 50 % and therefore increases the sustained dam-age for this type of buildings. This might cause a defi-ciency in a probable future earthquake and preventivemeasures should be taken urgently.

5. The magnitude of maximum inter-storey drift ratios andthe distribution of this ratio over the height of the allstructures are very similar since the effects of highermodes are negligible and the response is primarily gov-erned by the fundamental mode.

6. Recalling that these template structures were designedand constructed according to force based design ap-proximations at the date of construction, such kind ofdeformation based deficiency (reduction of wall thick-ness and its effects on performance) may not be cap-tured by means of force based evaluation procedures.On the other hand, performance based seismic assess-ment procedures are useful tools to correctly predict thedeficiencies in this type of masonry constructions as inthe case of framed structures.

7. In this study, two types of damage limits state defini-tions suggested by Calvi (1999) and Lagomarsino andPenna (2003) for the performance assessment of build-ings have been taken into consideration. In the firstapproach, inter-storey drift ratios are used as damagelimit states, whereas second one is useful if the assess-ment procedure is limited for global response predic-tion. For the studied buildings, while the second ap-proach can give useful information for the global re-sponse of the buildings, it is incapable of representingthe effects of change in wall thickness and its effects onseismic performance. Considering that all the templatedesigns have wall thickness reduction from second tothird story, first method seems more convenient for cap-turing such kind of deficiencies.

8. The observed building damages for URM structuresduring the past earthquakes support the analytical re-sults obtained in this study; the reports and studies(Jagadish et al., 2003; Kaplan et al., 2010; Klingner,2006; Tomazevic, 1999; Yoshimura and Kuroki, 2001)

from past earthquakes pointed out masonry facadeswith numerous spandrels and between those spandrelsfailed due to shear. Stress concentrations due to shearin spandrels observed in pushover analyses for theseconstructions are clear indicators of such failures andpotential risk in existing URM buildings for futureearthquakes.

9. Shear failure of masonry piers seems the most frequentfailure mechanism of URM buildings in the past earth-quakes and the pushover analyses results support thisfact. However, with regard to the ductility and energydissipation capacity this mechanism is not favourable.Non-ductile behaviour of weak piers could be improvedby means of adequately distributed bed joint reinforce-ments.

10. According to performance evaluation, template designs,except the TD 72/1 building, are far from satisfyingthe expected performance levels, suggesting that urgentplanning and response need to be in action.

Acknowledgements.The authors would like to acknowledge thesupport provided by Epoka University. Appreciation is extendedto reviewers for their invaluable positive contribution for theimprovement of the paper.

Edited by: M. E. ContadakisReviewed by: N. Shkodrani and one anonymous referee

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